Molten rock can sometimes form huge reservoirs called magma chambers within the Earth's crust. Left undisturbed over many hundreds of thousands of years this magma will cool and crystallise to form intrusive igneous rocks.

Intrusive igneous rocks like granite and gabbro have some things in common. They:

1. Are large grained - magma cools very slowly beneath the Earth's surface so the crystals in the rock have a long time to grow.